Method of changing the resonant frequency of a quartz crystal



Aug. 4, 1959 H. E. WENDEN 2,898,243

METHOD OF CHANGING THE RESONANT FREQUENCY OF A QUARTZ CRYSTAL 7 FiledMarch 14, 1958 INVENTOR, HENRY E. WENDEN ATTORNEY United States Patentce METHOD OF CHANGING THE RESONANT FRE- QUENCY OF A QUARTZ CRYSTAL HenryE. Wenden, Columbus, Ohio, assignor to the United States of America asrepresented by the Secretary of the Army Application March 14, 1958,Serial No. 721,612

8 Claims. (Cl. 117-223) This invention relates to quartz crystalresonators and particularly to methods of and means for adjusting theresonant frequency thereof to closer tolerances.

Prior to the present invention, in the manufacture of BT quartzoscillator plates, the final finishing of the plates to their desiredoscillating frequency required hand lapping or acid etching. In a platedesired to have a final frequence of, say 6,000 kc., thepre-hand-finishing treatment is stopped when the plate is roughly a fewhundred cycles up to a few kilocycles under the final desired frequency.To increase the frequency of the plate to a desired frequency by theseconventional lapping or etching techniques is a slow, tedious and costlyoperation at best and leaves much to be desired.

According to the present invention, the difficulties experienced withthe prior art have been overcome by subjecting the quartz oscillatorplates to the action of heat and an applied electric field to diffusetherein ions of a salt or oxide of an alkali metal or an alkaline earthmetal into the plates in an amount suflicient to change their frequencyof oscillation.

The change in frequency produced in oscillator plates by ion diffusiontreatment is downwards from the original value by as much as fourkilocycles when the plates are treated with ions of metals such asberyllium, magnesium, sodium or calcium. On the other hand, theoscillator plates may be shifted upward in frequency as much as twelvekilocycles by diffusion into them of alien ions of metals, such asbarium, cesium, lithium or strontium. The downward or upward change infrequency of oscillation of the plates appears to depend primarily onthe material used for the source of ions.

For a more detailed description of the invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the single figure of theaccompanying drawing, which is a vertical cross-sectional view of theapparatus embodying the invention.

The apparatus shown comprises a conventional mufile furnace 10 in whichare mounted the various elements of the quartz oscillator plate holder12, appropriately insulated from the furnace 10 and connected throughinsulated connections 30 and 32 to a source of direct current voltage34.

Inside of the furnace 10 is positioned a ceramic plate 14 which isolatesthe entire holder 12 from the furnace lining to prevent shunt pathsaround the oscillator plate 16. A graphite cylinder 18 is mounted on theceramic plate 14. Mounted on the graphite cylinder 18 is a graphiteblock 20 substantially narrower than the diameter of cylinder 18. Block20 has a central bore 22 which serves as well in which the material 24used as a source of ions is packed. The material 24 preferably consistsof a salt or an oxide of an alkali metal or an alkaline earth metal.Specific materials include beryllium sulphate, magnesium chloride,calcium fluoride, sodium chloride, calcium chloride, barium chloride,stron- Patented Aug. 4, 1959 2 tium fluoride, lithium chloride, cesiumchloride, magnesium oxide, strontium oxide and calcium oxide.

Quartz oscillator plate 16, larger in diameter than block 20, is mountedon block 20 and centered over the bore 22 so that it is in contact withmaterial 24. An other graphite cylinder 26 identical to the lowercylinder 18 is mounted on quartz plate 16. On top of cylinder 26 a heavycylinder 28 of stainless steel is placed and serves as a weight to holdthe above elements of the holder under compression. The ends of a pairof platinurn wires 30 and 32 are suitably secured to make electricalcontact to graphite cylinders 18 and 26, respectively.

In operation, the furnace 10 is heated to its operating temperature andthe direct current voltage is applied to the graphite cylinders 18 and26. When plate 16 acquires the desired oscillating frequency value orrange, furnace 10 is turned off and the connection to voltage source 34is opened.

In accordance with the invention, quartz oscillator plates were treatedfor periods up to 12 hours, at approximately 300 volts DC. potentialwith furnace temperatures ranging between 475 to 485 centigrade. Theoscillator plates decreased in frequency in the case of dilfusion withions of metals such as beryllium, magnesium, sodium or calcium. Whenbarium, cesium, lithium or strontium were used as ion source material,the frequency of oscillation increased.

In one experiment a BT-cut quartz oscillator plate was treated withbarium chloride at 475 C. and 300 volts D.C. After 12 hours thefrequency of the plate was found to have increased from 5925.1 kc. to5937.1 kc., showing an increase of 12 kc. In another experiment a BT-cutquartz oscillator plate was treated with sodium chloride at 485 C. and325 volts DC. After 12 hours it was found that the oscillating frequencyof the plate was decreased from 6573.6 kc. to 6569.2 kc., showing adecrease in frequency of oscillation of 4.4 kc. Similar results ofvarying degree were obtained with the other materials, and by variationof the amount and potential and the time of exposure thereof.

While there has been described what is at present considered a preferredembodiment of the invention, it will be obvious to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the invention, and it is therefore aimed in the appendedclaims to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. The method of adjusting the resonant frequency of a quartz crystalcomprising placing said crystal in contact with a compound selected fromthe group consisting of the inorganic salts and oxides of beryllium,magnesium, calcium, strontium, barium, lithium, sodium and cesium,maintaining said crystal and said compound at a predeterminedtemperature, and applying a direct current potential across said crystaland said compound for a predetermined period of time.

2. The method of changing the resonant frequency of a quartz crystalcomprising placing said crystal in contact with a compound selected fromthe group consisting of the inorganic salts of beryllium, magnesium,calcium, strontium, barium, lithium, sodium and cesium, maintaining saidcrystal and said compound at a predetermined temperature, and applying adirect current potential across said crystal and said compound for apredetermined period of time.

3. The method of increasing the oscillating frequency of a quartzoscillator plate comprising placing said plate in contact with aninorganic salt selected from the group consisting of barium, cesium,lithium and strontium, maintaining said plate and said salt at apredetermined temperature, and applying a direct current potentialacross said plate and said salt for a period of time to permit ions ofsaid salt to diffuse into said plate until a desired increase inoscillating frequency of said plate is effected.

4. The method of decreasing the oscillating frequency of a quartzoscillator plate comprising placing said plate in contact with aninorganic salt selected from the group consisting of beryllium,magnesium, sodium and calcium, maintaining said plate and said salt at apredetermined temperature, and applying a direct current potentialacross said plate and said salt for a period of time to permit ions ofsaid salt to diffuse into said plate until a desired decrease inoscillating frequency of said plate is eifected.

5. The method of increasing the resonant frequency of a quartz crystalcomprising placing said crystal in contact with an inorganic salt ofbarium, maintaining said crystal and said salt at a predeterminedtemperature, and applying a direct current potential across said crystaland said salt for a predetermined time.

6. The method as set forth in claim 5, wherein said inorganic salt ofbarium is barium chloride, said temperature is 475 C., and said time is12 hours, and said potential is 300 volts.

7. The method of decreasing the resonant frequency of a quartz crystalcomprising placing said crystal in contact with an inorganic salt ofsodium, maintaining said crystal and said salt at a predeterminedtemperature, and applying a direct current potential across said crystaland said salt for a predetermined time.

8. The method as set forth in claim 7, wherein said inorganic salt ofsodium is sodium chloride, said temperature is 485 C., and said time is12 hours, and said potential is 325 volts.

Williams Jan. 18, 1938 Moulton Nov. 1, 1949

1. THE METHOD OF ADJUSTING THE RESONANT FREQUENCY OF A QUARTZ CRYSTALCOMPRISING PLACING SAID CRYSTAL IN CONTACT WITH A COMPOUND SELECTED FORMTHE GROUP CONSISTING OF THE INORGANIC SALTS AND OXIDES OF BERYLLIUM,MAGNESIUM, CALCIUM, STRONTIUM, BARIUM, LITHIUM, SODIUM AND CESIUM,MAINTAINING SAID CRYSTAL AND SAID COMPOUND AT A PREDETERMINEDTEMPERATURE, AND APPLYING A DIRECT CURRENT POTENTIAL ACROSS SAID CRYLSTAAND SAID COMPOUND FOR A PREDETERMINED PERIOD OF TIME.